Abstract
Monte Carlo event generators for hard hadronic collisions depend on the evolution of parton showers backwards from a high-scale subprocess to the hadronization scale. The evolution is treated as a branching process with a sequence of resolvable parton emissions. The criterion of resolvability involves cutoffs that determine the no-emission probability (NEP) for a given range of the evolution scale. Existing event generators neglect cutoff-dependent terms in the NEP that, although formally power-suppressed, can have significant phenomenological effects. We compute such terms and study their consequences. One important result is that it is not possible for the backward shower to faithfully reproduce the cutoff-independent parton distribution functions (PDFs) used to generate it. We show that the computed NEP corrections mitigate but do not eliminate this problem. An alternative approach is to use cutoff-dependent PDFs that are consistent with the uncorrected NEP. Then one must apply cutoff-dependent corrections to hard subprocess matrix elements. We compute those corrections to the first nontrivial order for the Drell-Yan process and for Higgs production by gluon fusion.
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Acknowledgments
We are grateful to Torbjörn Sjöstrand and Mike Seymour for valuable comments on the manuscript. This work has been partially supported by UK STFC HEP Theory Consolidated grant ST/T000694/1. SF thanks the CERN TH division for the kind hospitality during the course of this work.
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ArXiv ePrint: 2309.15587
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Frixione, S., Webber, B.R. Correcting for cutoff dependence in backward evolution of QCD parton showers. J. High Energ. Phys. 2024, 150 (2024). https://doi.org/10.1007/JHEP03(2024)150
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DOI: https://doi.org/10.1007/JHEP03(2024)150